U.S. Pat. Nos. 4,076,929 and 4,569,978 describe an improved emulsion polymerization process for preparing VDF homopolymers and copolymers having improved flexibility. According to the aforesaid process, a comonomer for VDF is added in an emulsion polymerization mixture as part of an initial charge, or the comonomer is introduced continuously in a fixed ratio with respect to VDF. The comonomer may comprise HFP. The invention of U.S. Pat. No. 4,076,929 results in extrusion grade polymer resins having improved melt flow characteristics which may be utilized for the fabrication of objects having greater flexibility and impact resistance. These improvements are diminished as the melt viscosity of the polymer is decreased to 20 kilopoise or lower, measured at 232.degree. C. and 100 sec.sup.-1 (ASTM 3835), unless increasing amounts of HFP comonomer are utilized. However, as the amount of HFP in the polymer is increased, the DSC melting point, and thus the use of temperature of the polymer, decreases.
What is needed is a VDF/HFP copolymer having improved physical properties, particularly improved melt flow, which improved properties are obtained without significant reduction in polymer use temperature, as compared to the use temperature of VDF homopolymer. More particularly, there is a need for a VDF-based polymer which has improved melt flow, but which substantially maintains the melting point of VDF homopolymer.